The present invention generally relates to photoelectron image projection apparatuses, and more particularly to a photoelectron image projection apparatus which projects a pattern of a mask onto a wafer by irradiating photoelectrons released from the mask which is deposited with a photoelectric layer.
As a fine pattern technique of producing a semiconductor integrated circuit such as a very large scale integrated circuit (VLSI), there is an exposure technique in which a fine pattern is exposed on a wafer. A photoelectron image projection apparatus which uses photoelectron image projection has a high resolution, and active research is made on the new exposure technique as enabling a finer pattern projection. The photoelectron image projection apparatus has an advantage in that the through-put is high because it uses the projection system, and improvements on the photoelectron image projection apparatus are highly desired.
FIG. 1 generally shows a conventional photoelectron image projection apparatus, and FIG. 2 shows a mask which is used in the photoelectron image projection apparatus.
In FIG. 1, the image projection is carried out inside a projection chamber 1 under vacuum, and a wafer stage 3 and a mask holder 5 are arranged within the projection chamber 1. The wafer stage 3 holds a wafer 2 and is movable for changing the projection position on the wafer 2. The mask holder 5 holds a mask 4 at a position confronting in opposed relationship to the wafer 2. A high voltage is applied to the mask 4, relatively to the stage 3, by a high voltage source 6, and a magnetic field is applied, in a direction of from the mask 4 to the wafer 2, by coils 7. A light source 8 for emitting an ultraviolet light L and a shutter 9 are arranged outside a window 1a which is provided at a position in the wall of the projection chamber 1 above the mask 4.
As shown in FIG. 2, a patterned opaque layer 4b for blocking the ultraviolet light L is formed on a substrate 4a which transmits the ultraviolet light L. A photoelectric layer 4c which releases the photoelectrons when irradiated with the ultraviolet light L is deposited on the entire surface of the substrate 4a provided with the patterned opaque layer 4b.
The ultraviolet light L from the light source 8 is irradiated onto the mask 4 from the side of the substrate 4a when the shutter 9 is open, and the ultraviolet light L which is not blocked by the patterned opaque layer 4b reaches the corresponding, remaining portions of the photoelectric layer 4c, so that photoelectrons E are released from those portions of the photoelectric layer 4c receiving the ultraviolet light L. The photoelectrons E are accelerated by the electrical field and the magnetic field between the mask 4 and the wafer 2 and are converged onto the wafer 2. As a result, a pattern, formed by the portions of layer 4c which release the photoelectrons E, is projected onto the wafer 2.
The photoelectric layer 4c of the mask 4 uses a material which includes cesium as one of the elements and which a large photoelectric effect and cannot maintain a stable state in an atmospheric air environment. Hence, the photoelectric layer 4c is deposited by a vapor deposition under vacuum and then used for the projection without exposing the photoelectric layer 4c to the atmospheric air.
Accordingly, in FIG. 1, a vapor deposition chamber 11 for depositing the photoelectric layer 4c under vacuum is connected to the projection chamber 1. A vapor deposition source 12 of the photoelectric layer 4c is provided within the vapor deposition chamber 11.
In the conventional photoelectron image projection apparatus, the intensity distribution of the photoelectrons E within the projection pattern becomes non-uniform and causes a non-uniform projection of the pattern as a result of the distribution of the photoelectric layer 4c being non-uniform. For example, the non-uniform distribution of the photoelectric layer 4c may be caused by the deposition state of the photoelectric layer 4c, that is, the contamination of the photoelectric layer 4c and the irregular thickness of the photoelectric layer 4c. When the non-uniform projection of the pattern occurs, there are problems in that the obtained pattern is defective and this results in the loss of time and waste of material.
On the other hand, the mask 4 deposited with the photoelectric layer 4c inside the deposition chamber 11 is used as it is for the projection in the projection chamber 1. For this reason, a defect is generated in the obtained pattern when a foreign substance such as a dust particle adheres onto the mask 4 when being transported or subjected to the vapor deposition of the photoelectric layer 4c. Again, this results in the loss of time and waste of material.
Therefore, the conventional photoelectron image projection apparatus has no means of checking the projection pattern and a defect in the pattern can only be detected by actually checking the pattern formed on the wafer 2. Thus, when a defect is detected in the pattern formed on the wafer 2, the photoelectron image projection apparatus must, at once, be stopped to remove the cause of the defect, that is, to replace the defective mask 4 with a new mask, and the pattern formed on the next wafer must again be checked for defects. As a result, the production efficiency of the conventional photoelectron image projection apparatus is poor.